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Effects of Size on Collision Perception and Implications for Perceptual Theory and Transportation Safety

People intentionally avoid and create collisions. Drivers control the speed and position of their vehicle to avoid a collision with another vehicle. Perception of depth (perception of objects’ locations) and time-to-collision (perception of when a collision will occur) are heavily relied on and supported by different information sources. Depth cues, such as relative size, provide heuristics for relative depth, whereas optical invariants, such as tau, provide reliable time-to-collision information. There are different information sources to support the perception of depth and time-to-collision. One would expect people to rely on invariants rather than depth cues, but the size-arrival effect shows the contrary. The size-arrival effect has important implications for transportation safety and depends on effective perception of potential collision events. Drivers may risk collisions by turning left in front of small oncoming vehicles that appear further than the actual distance. Effects of vehicle size on time-to-collision judgments were demonstrated with computer simulation of a traffic scene that represented a left turn situation. Vehicles of different sizes approached an intersection and participants were asked to press a button when they thought the vehicle would reach them. The participants responded sooner for larger vehicles compared to smaller vehicles which is consistent with the size-arrival effect. Drivers may perceive that they have more time to complete a left turn when the oncoming vehicle is small, such as a motorcycle, than with a larger vehicle, such as a truck. The effect of size on collision perception raises important questions for perceptual theory. Effects of size on the perception of collision must be considered in the analysis of transportation accidents and in the design of countermeasures.